This invention relates to apparatus for detecting overload of a crane.
When an overload is applied to a crane there occurs such troubles as turn over of the crane, breakage of the boom and damage of the goods lifted by the crane. Accordingly, it is important for the crane operator to know the overload condition.
The factors that cause overloading of a crane are the weight of the goods lifted by the crane and the operating radius. However, there is no suitable location at which the weight of the goods can be detected. Even when the weight is detected its measured value has been extremely inaccurate so that if such detected value were used as a parameter for the apparatus for preventing overload, the accuracy of the apparatus would be extremely low.
According to one method of detecting the weight of a lifted load, a load cell is used to measure directly the weight. However, the range of the weight that can be measured by this method is not so wide. Since the weight of the load handled by a crane generally varies over a wide range of from 300 kg to 30 tons it is impossible to measure the weight at high accuracies. Further, where a load cell is used, unless the position on the crane at which the load cell is to be mounted is precisely selected, the detected value would contain an erroneous component.
According to another method, the tension of a rope for supporting the load is utilized to measure the weight of the load. With this method the sheave efficiency of the pulley affects directly the measured value thus causing it to be inaccurate. Although many other methods of measuring the weight of the load have been proposed, the accuracy of the measurement is decreased due to frictions of the pulley, pins, oil pressure packings or the like which are difficult to estimate.